[CH2:1]([N:3]([CH2:20][CH3:21])[C:4]1[CH:13]=[C:12]2[C:7]([CH:8]=[C:9]([C:15]([O:17]CC)=[O:16])[C:10](=[O:14])[O:11]2)=[CH:6][CH:5]=1)[CH3:2].[OH-].[Na+]>CO>[CH2:20]([N:3]([CH2:1][CH3:2])[C:4]1[CH:13]=[C:12]2[C:7]([CH:8]=[C:9]([C:15]([OH:17])=[O:16])[C:10](=[O:14])[O:11]2)=[CH:6][CH:5]=1)[CH3:21]--A solution of the ester 1 (11.6g, 40 mmol) in MeOH (100 ml) was heated under reflux and 0.5 M aq. NaOH (100 ml) was added rapidly. A yellow solid precipitated within 5 min and the mixture was cooled and acidified with 2 M aq. HCl to give an orange solid which was filtered and washed with 2 M aq. HCl, then with water until neutral and finally with MeOH (50 ml), and dried in vacuo to give the acid 2 (9.4 g) A sample recrystallised from MeOH-diisopropyl ether as orange laths, m.p. 231-232° C. (lit. m.p. 227-229° C. see U.S. Pat. No. 4,317,774).
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